The placement of concrete into forms is a critical phase of most commercial, highway, industrial and residential construction projects. Typically, construction contractors are required to place concrete into forms as soon as possible after mixing, while the concrete is in a plastic and workable state. In many projects it is also required that concrete placement be continuous to allow for appropriate consolidation of adjoining layers to allow formation of uniform, dense, impervious concrete with smooth faces on exposed surfaces. The failure to properly place concrete in conformance with such exacting standards can result in structurally defective concrete that in many instances must be replaced at great cost to the construction contractor.
The use of concrete pumps for the placement of concrete into forms has gained widespread acceptance as a safe, efficient, and cost effective method of concrete placement. Concrete pumping allows for direct placement of concrete into forms without rehandling, resulting in significantly reduced labor costs and improved concrete quality. Generally, the use of concrete pumps is faster than traditional methods, because pumping allows for continuous placement of concrete. This can increase the productivity of finishers and decrease total construction time for the overall project, minimizing overhead costs and monetary penalties associated with construction delays.
Placement of concrete utilizing the pumping method involves the flow of concrete from a pump through a conduit, or pump line, such as flexible hoses or metal pipe. Concrete exits the pump line in a precisely controlled manner directly into forms. Pumping concrete offers a steady, predictable flow of concrete delivered to the point-of-use resulting in a smooth-flowing operation. A significant concern in concrete pumping operations is ensuring that the inner, concrete-contacting surfaces of the pump line are lubricated to facilitate the flow of concrete therethrough. This is not a problem after the flow of concrete has been established, because as it passes through a pump line the concrete acts as a self-lubricating agent by leaving a layer of slick cement slurry on the inner walls of the pump line. This is a problem, however, in initially establishing concrete flow in a pump line. Attempting to pump concrete through a dry pump line will cause the pump line to plug, resulting in costly construction delays. To address this problem, concrete pumpers have traditionally added water to the first batch of concrete to allow it to pump easier. This conventional, “just wetting” technique is unacceptable because in most cases it does not prevent the pump line from plugging. Further, the introduction of excess water into the mixed concrete alters the target water-cement ratio of the concrete, causing decreased strength of the finished concrete and segregation of the components of the concrete. To address the problem, the American Concrete Pumping Association recommends always priming a concrete pump and line prior to initiating concrete pumping.
Priming a concrete pump line involves the application of a lubricating agent to the inner walls of the pump line to promote the establishment of concrete flow through the line. This is typically accomplished by pumping a priming fluid through the pump line immediately prior to initiating the pumping and flow of concrete through the pump line. The priming fluid is of a type that is easily pumped through a dry concrete pump line, and that will lubricate the inner, concrete-contacting surfaces of the pump line as it is pumped therethrough. In practice, the priming fluid once prepared is placed in fluid communication with the suction side of a concrete pump. In practice, the priming fluid is often placed in a vessel, referred to as a hopper, that is in fluid communication with the concrete pump. As the priming fluid is being pumped from the hopper, concrete is typically added to the hopper such that the final portion of the priming fluid is in contact with the initial portion of the concrete, with the priming fluid thereby preceding the concrete that is pumped through the concrete pump line. The priming fluid acts to lubricate the inner, concrete-contacting surfaces of the concrete pump line to facilitate the initial movement of concrete therethrough.
Various compositions for priming concrete pumps and pump lines are known in the prior art, however, all are disadvantageous when compared to the present invention. For instance, one method of priming concrete pumps utilizes ready-mixed priming grout. This priming grout slurry has the disadvantage of being expensive, requiring the use of a ready-mix truck. This method also has the disadvantage of utilizing large volumes of priming grout slurry that cannot be mixed with concrete, i.e., added to the form and mixed with concrete. This method therefore produces a large volume of priming grout slurry waste that requires offsite disposal. Another method of priming concrete pumps utilizes a concrete priming slurry, consisting of cement and water. Although the concrete priming slurry may be prepared without the use of a ready-mix truck, this concrete priming slurry method has the disadvantage of requiring the onsite delivery and handling of extra bags of cement. Additionally, similar to the priming grout slurry, the concrete priming slurry is a waste that requires offsite disposal.
The prior art also teaches the use of specialized lubricants as a priming fluid. For instance, U.S. Pat. No. 5,997,633 to Montgomery teaches the use of a priming fluid comprising an additive mixed with water, the additive having a composition by weight of 80 to 90 percent alkaline material and 10 to 20 percent polymeric material. Such a priming fluid presents disadvantages, as the strongly alkaline nature of the priming fluid, having a pH of between 11 and 12, presents safety concerns associated with the handling of the priming fluid. Further, the strongly alkaline nature of the priming fluid increases the risk of producing weak or unstable set concrete as a result of alkali-silica reactivity and alkali-aggregate reactivity. Other disadvantages with the use of alkaline priming fluid are associated with the cost of the alkaline materials and the relatively long duration of additive mixing and hydration. The use of the priming fluid prior to full hydration presents the risk of drawing water from concrete as it passes through a pump line, which interferes with the desired lubricating effect of the priming aid.
U.S. Pat. No. 5,318,408 to Davidsson teaches the use of an aqueous suspension, in contrast to an aqueous solution, as a concrete priming fluid. This reference discloses the use of an aqueous suspension comprising amorphous silica, namely silica fume, and a water dispersable polymer consisting of vinyl esters, acrylic acid esters, styrene, butadiene, and vinyl-halogen compounds. The use of such suspensions instead of aqueous solutions as a priming fluid presents several disadvantages. For instance, the suspension becomes unpumpable if subjected to pumping pressures higher then the segregation pressure required to separate the solids and liquids in the suspension. Therefore, the user must carefully monitor the pumping pressure to ensure that it does not exceed the suspension's segregation pressure. Further, the limitation of using water dispersable polymers provides a pump primer having inferior lubricating properties. Other disadvantages with the use of an aqueous suspension are associated with the difficulty of maintaining a uniform suspension during use, which interferes with the lubricating effect of the pumping aid.
Accordingly, it is recognized in the construction industry that there is an increasing need for improved compositions for and methods of priming a concrete pump line to lubricate the pump line to promote the initial flow of concrete through the line. The present invention addresses the need for improved compositions for use in priming concrete pump lines in commercial, highway, industrial and residential construction projects